ReadDirectoryChangesW

The ReadDirectoryChangesW function returns information describing the changes occurring within a directory.

BOOL ReadDirectoryChangesW(
  HANDLE hDirectory,    // handle to the directory to be watched
  LPVOID lpBuffer,      // pointer to the buffer to receive the read 
                        // results
  DWORD nBufferLength,  // length of lpBuffer
  BOOL bWatchSubtree,   // flag for monitoring directory or 
                        // directory tree
  DWORD dwNotifyFilter, // filter conditions to watch for
  LPDWORD lpBytesReturned,  // number of bytes returned
  LPOVERLAPPED lpOverlapped,
                        // pointer to structure needed for 
                        // overlapped I/O
  LPOVERLAPPED_COMPLETION_ROUTINE lpCompletionRoutine
                        // pointer to completion routine
);
 

Parameters

hDirectory

Handle to the directory to be watched. This directory must be opened with the FILE_LIST_DIRECTORY access right.

lpBuffer

Specifies the address of the formatted buffer in which the read results are to be returned. The structure of this buffer is defined by the FILE_NOTIFY_INFORMATION structure. This buffer is filled either synchronously or asynchronously, depending on how the directory is opened and what value is given to the lpOverlapped parameter. For more information, see the Remarks section.

nBufferLength

Specifies the length of the buffer pointed to by the lpBuffer parameter.

bWatchSubtree

Specifies whether the ReadDirectoryChangesW function will monitor the directory or the directory tree. If TRUE is specified, the function monitors the directory tree rooted at the specified directory. If FALSE is specified, the function monitors only the directory specified by the hDirectory parameter.

dwNotifyFilter

Specifies filter criteria the function checks to determine if the wait operation has completed. This parameter can be one or more of the following values:

Value	Meaning
FILE_NOTIFY_CHANGE_FILE_NAME	Any filename change in the watched directory or subtree causes a change notification wait operation to return. Changes include renaming, creating, or deleting a file.
FILE_NOTIFY_CHANGE_DIR_NAME	Any directory-name change in the watched directory or subtree causes a change notification wait operation to return. Changes include creating or deleting a directory.
FILE_NOTIFY_CHANGE_ATTRIBUTES	Any attribute change in the watched directory or subtree causes a change notification wait operation to return.
FILE_NOTIFY_CHANGE_SIZE	Any file-size change in the watched directory or subtree causes a change notification wait operation to return. The operating system detects a change in file size only when the file is written to the disk. For operating systems that use extensive caching, detection occurs only when the cache is sufficiently flushed.
FILE_NOTIFY_CHANGE_LAST_WRITE	Any change to the last write-time of files in the watched directory or subtree causes a change notification wait operation to return. The operating system detects a change to the last write-time only when the file is written to the disk. For operating systems that use extensive caching, detection occurs only when the cache is sufficiently flushed.
FILE_NOTIFY_CHANGE_LAST_ACCESS	Any change to the last access time of files in the watched directory or subtree causes a change notification wait operation to return.
FILE_NOTIFY_CHANGE_CREATION	Any change to the creation time of files in the watched directory or subtree causes a change notification wait operation to return.
FILE_NOTIFY_CHANGE_SECURITY	Any security-descriptor change in the watched directory or subtree causes a change notification wait operation to return.

lpBytesReturned

For synchronous calls, this parameter specifies the number of bytes transferred into the lpBuffer parameter. For asynchronous calls, this parameter is undefined. You must use an asynchronous notification technique to retrieve the number of bytes transferred.

lpOverlapped

Pointer to an OVERLAPPED structure that supplies data to be used during asynchronous operation. Otherwise, this value is NULL. The Offset and OffsetHigh members of this structure are not used.

lpCompletionRoutine

Pointer to a completion routine to be called when the operation has been completed or canceled and the calling thread is in an alertable wait state. For more information about this completion routine, see FileIOCompletionRoutine.

Return Values

If the function succeeds, the return value is nonzero. For synchronous calls, this means that the operation succeeded. For asynchronous calls, this indicates that the operation was successfully queued.

If the function fails, the return value is zero. To get extended error information, call GetLastError.

Remarks

To obtain a handle to a directory, use the CreateFile function with FILE_FLAG_BACKUP_SEMANTICS as follows:

hDir = CreateFile(
  DirName,                            // pointer to the file name
  FILE_LIST_DIRECTORY,                // access (read-write) mode
  FILE_SHARE_READ|FILE_SHARE_DELETE,  // share mode
  NULL,                               // security descriptor
  OPEN_EXISTING,                      // how to create
  FILE_FLAG_BACKUP_SEMANTICS,         // file attributes
  NULL                                // file with attributes to copy
);
 

A call to ReadDirectoryChangesW can be completed synchronously or asynchronously. To specify asynchronous completion, open the directory with CreateFile as shown above, but additionally specify the FILE_FLAG_OVERLAPPED attribute in the dwFlagsAndAttributes parameter. Then specify an OVERLAPPED structure when you call ReadDirectoryChangesW.

Upon successful synchronous completion, the lpBuffer parameter is a formatted buffer and the number of bytes written to the buffer is available in lpBytesReturned. If the number of bytes transferred is zero, the buffer was too small to provide detailed information on all the changes that occurred in the directory or subtree. In this case, you should compute the changes by enumerating the directory or subtree.

For asynchronous completion, you can receive notification in one of three ways:

• Using the GetOverlappedResult function. To receive notification through GetOverlappedResult, do not specify a completion routine in the lpCompletionRoutine parameter. Be sure to set the hEvent member of the OVERLAPPED structure to a unique event.
• Using the GetQueuedCompletionStatus function. To receive notification through GetQueuedCompletionStatus, do not specify a completion routine in lpCompletionRoutine. Associate the directory handle hDirectory with a completion port by calling the CreateIoCompletionPort function.
• Using a completion routine. To receive notification through a completion routine, do not associate the directory with a completion port. Specify a completion routine in lpCompletionRoutine. This routine is called whenever the operation has been completed or canceled while the thread is in an alertable wait state. The hEvent member of the OVERLAPPED structure is not used by the system, so you can use it yourself.

QuickInfo

  Windows NT: Requires version 4.0 or later.
  Windows: Unsupported.
  Windows CE: Unsupported.
  Header: Declared in winbase.h.
  Import Library: Use kernel32.lib.

See Also

File I/O Overview, File Functions, CreateFile, CreateIoCompletionPort, FILE_NOTIFY_INFORMATION, FileIOCompletionRoutine, GetOverlappedResult, GetQueuedCompletionStatus, OVERLAPPED